Substantial effort and expense have been devoted to the development of optical memories, primarily in an attempt to improve the storage capacity-to-physical size relationship of random access mass data storage systems. A significant portion of this work has been directed toward so-called direct read after write optical memories because they are compatible with relatively straightforward error direction and correction techniques.
Optical disk memories are attractive for mass data storage applications because they offer relatively high data packing densities. However, this advantage is reduced, or even nullified, if there is an unacceptably high data error rate. As will be appreciated, more or less conventional error correction techniques may be employed to correct for random isolated errors and for relatively short burst errors. Accordingly, an unacceptably high data error rate is normally associated with longer burst errors, such as may be caused by a defect in the recording medium or by drift in the timing of the data recording process. Of course, it is essential to limit the data error rate to an acceptably low level because otherwise the record (i.e., the recorded data) may be useless.
Direct read after write memories have the advantage that the recorded data can be directly verified. If the verification process indicates that the data error rate is running at an unacceptably high level, the data can be re-recorded, typically on another sector of the recording medium. That usually is an acceptable solution to the problem if the elevated error rate is attributable to, say a localized defect in the recording medium. However, it is of little, if any, assistance if the increased error rate is caused by a loss of timing in the data writing process. A well designed system generally has some margin for such timing errors, but there still is the risk that changes in the operating characteristics of the system will produce timing errors which exceed those margins.
Others have addressed some of the problems that may occur in optical memories due to time dependent changes in their operating characteristics. For example, U.S. Pat. No. 4,093,961 on "Optical Reading Apparatus with Scanner Light Intensity Control" relates to a feedback system for adjusting the output power of a laser in order to maintain a substantially constant average optical intensity level at a data detecting/tracking photodetector. Unfortunately, such read or sensor channel compensation techniques cannot correct for errors caused by drift or other variations in the data recording parameters. Somewhat more to point, it has been suggested that the write channel performance should be periodically adjusted, say, once per sector, as necessary to cause a recorded test pattern to conform to a predetermined profile. However, that is a relatively slow process which only partially compensates for time dependent changes in the operating characteristics of the write channel.